This invention relates to seismic signal processing generally and specifically relates to the identification and attenuation of non-primary events in seismic data.
In seismic data processing, identification and attenuation of non-primary events (e.g. multiples, ghosts, etc.) is desirable. One specific example of such efforts is the so-called multiple diffraction attenuation (MDA), which is a processing technique that has been designed by PGS Data Processing to attenuate high-energy reverberations of near-surface diffractions. Such reverberations are commonly seen on deep-water datasets with complex, shallow sub-seabed topography such as those recently acquired by PGS offshore Nigeria and in the South Rockall Trough.
Conventional MDA processing is based on the assumption that, at greater depths, multiples tend to have significantly higher amplitudes at higher frequencies than primaries do. The reason for this is greater primary absorption that is caused by (1) increased wave-field spreading (longer travel-paths) and (2) higher absorption. FIG. 1 illustrates the conventional procedure. In the high frequency constituent of the data, high amplitudes HA are identified by comparing amplitudes to the rms amplitude in a user-defined window. The boundaries of the window are dependant upon the specific shapes of the events expected bases on a priori knowledge and are within the skill of those in the art. For example, when amplitudes are higher than, say, two times the rms amplitude, then these amplitudes are zeroed. After stacking the high and low frequency constituents, the conventional MDA result is obtained. Although this method has proven to be an effective noise removal tool, the major disadvantage of the method is that it cannot separate high-amplitude primaries from high-amplitude multiples. As a result, high-amplitude primaries may be partly muted, as indicated by the discontinuous primaries P. Clients often recognize this disadvantage; and, as a result, the method is often not applied.
Therefore, there is a continuing need for a method of identification non-primary events, and it is an object of the present invention to provide for that need.
According to one aspect of the present invention, a method is provided for identifying primary seismic events in seismic data, the method comprising:
applying a coherency filter to the seismic data;
sorting the data according to an event characteristic having a tendency to separate primary from non-primary events; and
selectively attenuating according to the characteristic and the coherency of the events.
According to a more specific example of the present aspect, the coherency filter is applied in windows, the characteristic comprises amplitude in a limited range of frequencies, and the attenuation comprises reduction of amplitude (e.g. muting).
According to another broad aspect of the invention a system is provides for identifying primary seismic events in seismic data, the method comprising:
means for applying a coherency filter to the seismic data;
means for sorting the data according to an event characteristic having a tendency to separate primary from non-primary events; and
means for selectively attenuating according to the characteristic and the coherency of the events.
According to still another aspect of the present invention, there is provided a method of identification of primary events in seismic data, the method comprising:
sorting the data by frequency wherein at least some non-primary events are separated from primary events, wherein a frequency-sorted gather of data results;
attenuating in the frequency-sorted gather amplitudes above a pre-selected base amplitude, wherein attenuated amplitudes result;
applying a coherency filter to the events, wherein coherent events are identified;
replacing with amplitudes from the coherent events attenuated amplitudes in the frequency-sorted gather corresponding to the coherent events.
According to a more specific example of the present aspect, the attenuating comprises reducing amplitude. According to an alternative example, the attenuating comprises muting.
According to another aspect of the present invention, there is provided a method of identification of primary events in seismic data, the method comprising:
sorting the data by frequency wherein at least some non-primary events are separated from primary events, wherein a frequency-sorted gather of data results;
applying a coherency filter to the events, wherein coherent events are identified;
attenuating in the frequency-sorted gather amplitudes above a pre-selected base amplitude which are not associated with the coherent events, wherein attenuated amplitudes result.
According to a more specific example of the present aspect, the attenuating comprises reducing amplitude. According to an alternative example, the attenuating comprises muting.
According to yet another aspect of the present invention, there is provided a system of identification of primary events in seismic data, the method comprising:
means for sorting the data by frequency wherein at least some non-primary events are separated from primary events, wherein a frequency-sorted gather of data results;
means for attenuating in the frequency-sorted gather amplitudes above a pre-selected base amplitude, wherein attenuated amplitudes result;
means for applying a coherency filter to the events, wherein coherent events are identified;
means for replacing with amplitudes from the coherent events attenuated amplitudes in the frequency-sorted gather corresponding to the coherent events.
According to a more specific example of the present aspect, the means for attenuating comprises means for reducing amplitudes. According to another example, the means for attenuating comprises means for muting.
According to still a further aspect of the present invention, a system of identification of primary events in seismic data is provided, the system comprising:
means for sorting the data by frequency wherein at least some non-primary events are separated from primary events, wherein a frequency-sorted gather of data results;
means for applying a coherency filter to the events, wherein coherent events are identified;
means for attenuating in the frequency-sorted gather amplitudes above a pre-selected base amplitude which are not associated with the coherent events, wherein attenuated amplitudes result.
According to a more specific example of the present aspect, the means for attenuating comprises means for reducing amplitude. According to an alternative example, the means for attenuating comprises means for muting.